Microstructures and Some Properties of Fe-28%Cr-4%Mo Alloys

Abstract

Abstract The effect of heat treatment on the microstructure and properties of 28Cr-4Mo and 28Cr-4Mo-2Ni alloys with low carbon and nitrogen levels was investigated. Sigma and chi phases form in these alloys in the range 1300 to 1700 F (704 to 927 C). The rate of formation is slow. Heating 1 hour at 1500 F (816 C) results in only a small amount of sigma phase at grain boundaries. This does not impair corrosion resistance. Annealing at 2000 F (1093 C) readily dissolves this sigma phase, and also much larger amounts of sigma and chi phases formed after heating 100 hours at 1500 F. These alloys are subject to embrittlement by hydrogen. It was shown that embrittlement produced in cold worked material by brief cathodic charging in 10% H2SO4 containing a sulfide can be eliminated by exposure to boiling water or aging at room temperature. Both compositions showed excellent resistance to crevice corrosion in a 9 month test in the ocean at the New Jersey shore. Resistance to caustic cracking was demonstrated by a test in 50% NaOH at 200 C (392 F). A Type 304, 18Cr-8Ni stainless steel was severely cracked in this test with almost no general corrosion. The 28Cr-4Mo alloy was not cracked but was severely attacked by general corrosion, and the 28Cr-4Mo-2Ni alloy resisted cracking and was also resistant to general corrosion.